1. Field of the Invention
The present invention relates to a stencil printer, and more particularly, to a structure for preventing an ink leakage from a printing drum.
2. Description of the Prior Art
Stencil printers having such a basic construction as comprising a printing drum having a perforated construction in a circumferential portion thereof excluding two annular edge portions at opposite ends of a cylindrical configuration and a stencil sheet leading end mounting bar portion extending between the two annular edge portions along a generatrix of the cylindrical configuration, an inking roller provided inside the printing drum to supply ink to the perforated circumferential portion of the printing drum from the inside thereof, and a back press roller, wherein the printing drum and the back press roller are arranged close and in parallel to one another to be rotated mutually in opposite rotational directions, so as to apply a stencil printing onto a print sheet transferred through a nip region between the printing drum and the back press roller, have been proposed by Japanese Patent Application 63-28553 (Laid-open Publication 1-204781), Japanese Patent Application 1-47029 (Laid-open Publication 2-225078), Japanese Patent Application 2-223550 (Laid-open Publication 4-105984), etc., filed by the same assignee as the present application
Further, it has been proposed by Japanese Patent Application 3-162218 (Laid-open Publication 4-361043) filed by the same assignee as the present application to provide a print sheet leading end holding clamp in the back press roller of a stencil printer of the above-mentioned basic construction to accomplish a stencil printing capable of producing a clear print image without causing a back contamination of the print sheet by the ink of the print image of an adjacent print sheet when the printed sheets are stacked one over the other, according to the principle that the print sheet is pulled apart from the perforated portion of the printing drum while the inking roller is inhibiting any movement of an ink layer formed in the perforated portion of the print sheet. Still further, it has been proposed by Japanese Patent Application 7-214075 (Laid-open Publication 9-39359) filed by the same assignee as the present application to mount an elastic strip sheet at an outer circumferential surface of the back press roller along a generatrix thereof to prevent the ink from leaking into a clearance formed in an outer circumferential portion of the back press roller for receiving a rear edge portion of the clamp when it is inclined rearward to open, particularly even when the clamp laps over the perforated portion of the printing drum according to a relative rotational biasing between the printing drum and the back press roller for a longitudinal or up/down positional adjustment of a print image on a print sheet as proposed to be available by Japanese Patent Application 5-306033 (Laid-open Publication 7-137419) by the same assignee as the present application.
The accompanying FIG. 1 is a diagrammatical view showing an already publicly known basic construction of a stencil printer comprising a printing drum, an inking roller and a back press roller having a print sheet leading end holding clamp of the above-mentioned constructions. In the figure, 10 is the printing drum, and 14 is the back press roller. The printing drum 10 has axially opposite end portions formed of a pair of annular portions 16 which are connected with one another by a transverse bar portion 18 extending in parallel with the central axis of the printing drum, so as to construct a frame of the printing drum. A flexible perforated sheet 20 having a rectangular configuration in development is mounted to the frame with its opposite side edge portions being placed on the outer circumferential surfaces of the pair of annular portions 16, while its leading end portion 20a and its trailing end portion 20b are respectively mounted to the bar portion 18. Although in the figure the mounting of the leading end portion 20a and the trailing end portion 20b of the flexible perforated sheet to the bar portion 18 is diagrammatically shown as they are simply laid one over the other at a portion thereof, some particular constructions with respect to the mounting of the trailing end portion 20b to the bar portion 18 are shown in the above-mentioned Japanese Patent Application 1-47029 (Laid-open Publication 2-225078) and other Japanese patent applications such as Japanese Patent Application 5-306028 (Laid-open Publication 7-137415) and Japanese Patent Application 5-306029 (Laid-open Publication 7-137416) filed by the same assignee as the present application. A stencil sheet 19 is wrapped around the cylindrical surface of the printing drum 10 formed by the flexible perforated sheet 20 in a condition that its leading end is held by a clamp 21 of the above-mentioned type.
The leading end portion 20a and the trailing end portion 20b of the flexible perforated sheet 20 have each a non-perforated construction, while a central portion 20c has a perforated construction to let ink pass thereacross. A stencil leading end mounting portion 10a of a non-perforated construction in the form of a strip bar extending between the opposite ends of the printing drum 10 along a generatrix thereof is formed by a combination of the non-perforated leading and trailing end portions 20a and 20b of the flexible perforated sheet 20 and the bar portion 18.
On the other hand, the back press roller 14 is formed with a transverse groove 22 extending in parallel with the central axis thereof along a generatrix thereof. The printing drum 10 and the back press roller 14 are common in the size of diameter and are rotated mutually in opposite rotational directions in synchronization with one another in such a manner that the stencil sheet leading end mounting portion 10a of the printing drum and the transverse groove 22 of the back press roller align with one another. When viewed in FIG. 1, the printing drum rotates in the counter-clockwise direction, while the back press roller rotates in the clockwise direction.
Inside the printing drum 10, an inking roller 12 is mounted by a shaft 13 to be rotatable about its central axis, with its outer circumferential surface contacting the inner circumferential surface of the printing drum 10. In order to avoid that the inking roller 12 impulsively contacts the transverse bar portion 18 during rotations of the printing drum, a pair of cams 24 are provided along the pair of annular portions 16, while a pair of annular cam followers 23 to engage the cams are provided on the shaft 13 of the inking roller 12.
A clamp 25 is provided in the back press roller 14 adjacent to a rear edge of the transverse groove 22 as viewed along the rotational direction of the back press roller, to clamp the leading end of a print sheet onto the back press roller 14. By this arrangement, each print sheet is mounted on the back press roller 14 in such a manner that it is held at the leading end by the clamp 25, while the print sheet is transferred through a nip region between the press roller and the printing drum, starting from the clamped leading end, as the back press roller 14 rotates in the clockwise direction, with a print image being applied thereon by the ink supplied by the inking roller 12 to the inside of the flexible perforated sheet 20 and passed through the perforations of the flexible perforated sheet 20 and perforations of the stencil sheet 19.
In the case where the above-mentioned longitudinal or up/down positional adjusting means for the print image relative to the stencil sheet proposed by the above-mentioned Japanese Patent Application 5-306033 (Laid-open Publication 7-137419) is incorporated, when the printing drum 10 and the back press roller 14 are set at a standard condition with respect to the relative rotational position therebetween, a border point (actually a line) 26 between the perforated portion 20c and the non-perforated leading end portion 20a of the flexible perforated sheet is in alignment with a point (actually a line) 28 on the outer circumferential surface of the back press roller 14, and similarly, a border point (actually a line) 32 between the perforated portion 20c and the non-perforated rear end portion 20b of the flexible perforated sheet is in alignment with a point (actually a line) 34 on the outer circumferential surface of the back press roller 14. When the relative rotational position between the printing drum 10 and the back press roller 14 is varied for an adjustment of the longitudinal or up/down position of a print image, the corresponding points (lines) 28 and 34 on the back press roller 14 bias as shown by arrows 38 and 40, respectively, so that there can occur a condition that the perforated portion 20c of the flexible perforated sheet laps over the clamp 25.
In more detail, when there occurs the condition that the perforated portion 20c of the flexible perforated sheet laps over the clamp 25, since a relatively large clearance 27 is formed along the rear edge of the clamp 25 holding the leading end of a print sheet S as shown in FIG. 2 to allow for an inclining movement of the clamp, when the ink of the ink layer L in this region is pressed by the inking roller 12 from the inside of the perforated portion 20c, the ink is pushed out through the perforations of the perforated portion 20c to the inner side of the stencil sheet 19 with no sufficient back support for the stencil sheet, so that a substantial amount of ink will be pressed into a space between the perforated portion 20c and the stencil sheet, thus locally bulging the stencil sheet outward. When such a pressing out of the ink has once occurred, the pressed out ink does not return by itself to the inside of the perforated portion 20c even after the ink pressing action of the inking roller was cancelled, because the ink has a relatively high viscosity. Therefore, when such a phenomenon is repeated according to the rotations of the printing drum, the ink flows unidirectionally from the inside to the outside of the perforated portion 20c thereacross, accumulating between the outer circumferential surface of the printing drum and the stencil sheet. As will be clear from FIG. 2, such a leaking out of the ink due to the lack of back pressing at the clearance 27 occurs in the same manner even in the printer in which the perforated portion 20c has a rigid perforated construction. When such an ink accumulation progresses, there not only occurs an ink contamination therearound but, when it progresses much, the stencil sheet is strongly pressed against an edge of the clamp or the clamp mounting opening, thereby causing a damage of the stencil sheet, probable to proceed so far as to cause a breakage of the stencil sheet.
In order to meet with such a problem, in Japanese Patent Application 7-214075 (Laid-open Publication 9-39359) filed by the same assignee as the present application, it has been proposed to mount an elastic strip sheet 52 at a portion of the outer circumferential surface of the back press roller 14 along a generatrix thereof so as to cover the clamp 25 as shown in FIG. 3.
Although the elastic strip sheet 52 according to the former proposal generally accomplishes the object intended, the elastic strip sheet 52 is liable to a flexing, so that when the printer is operated for a long time in the condition that the perforated portion 20c of the flexible perforated sheet is laid over the clamp 25, it is apprehended that there still occurs an ink leakage by the same mechanism as described with reference to FIG. 2, due to a flexing of the elastic strip sheet 52.